(1) Field of the Invention
This invention relates to an image fixing device as for a copying apparatus which lets a copy sheet with a toner image formed thereon pass between a fixing roll, heated from inside by a heating member, and a pressing roll maintained in pressing engagement with the fixing roll, to fix the toner image by heating and pressing.
(2) Description of the Prior Art
One type of image fixing device for an electrophotographic copying machine and the like comprises a fixing roll having a heating member, such as an infrared heater, for heating the fixing roll from inside or which is composed of a metallic core directly heated by induction heating, a resilient material layer and an offset preventing layer superposed one over another in the indicated order, and a pressing roll juxtaposed against the fixing roll for pressing a thereagainst. By passing toner image supporting copy sheet between the fixing roll and pressing roll, the toner image is pressed against the outer periphery of the fixing heated roll, to cause the toner image adhere to the copy sheet by melt adhesion.
The resilient material layer, which is provided for bringing the copy sheet into contact with the fixing roll with a suitable degree of resilience and a width of contact, is usually formed of a resilient material of high resistance to heat, such as silicone rubber, fluoroprene rubber, etc., and has a thickness in the order of several millimeters. As subsequently to be described, the resilient material layer should have a sufficiently high thermal capacity to be commensurate with the thermal capacity of the toner image. In order to avoid offsetting of the toner image or adhering of the toner image to the surface of the fixing roller, the offset preventing layer which is superposed over the resilient layer, is formed of a material of high parting capability, such as silicone RTV rubber (KE12, a trade name produced by Sinetsu Chemical Company. Ltd.), fluoride resin (Teflon or polytetrafluoroethylene), etc., and has a small thickness of several scores of microns.
In selecting the material for forming the resilient material layer, it has hitherto been customary to make it a rule to select a material of suitable resilience and high resistance to heat. In selecting the material for the offset preventing layer, it has been customary to select a material of high parting capability and high resistance to heat, but little attention, if any, has ever been paid to the property of a response to heat of materials. Owing to the fact that this property of materials has not been taken into consideration, the image fixing devices of the prior art have had the disadvantage that as a copy sheet is brought into contact with the fixing roll in carrying out fixing, the temperature of the surface of the roll drops, or when fixing a plurality of toner images is carried out continuously, the temperature at the surface of the roll gradually drops, resulting in a lack of heat of an amount required for effecting fixing. When such phenomenon occurs, it has been necessary to raise the temperature of the heating member.